The present invention relates to a magnetic tape cassette, and more particularly to a magnetic tape cassette, used for a VTR, for example, with metal tape guides for guiding a magnetic tape while in contact with the magnetic surface of the magnetic tape.
Magnetic recording media have widely been used for recording tape cassettes, video tape cassettes, computer tape cassettes, disc cartridges, and the like. The recording density of a magnetic recording medium such as a magnetic tape has been increased year by year. With this increase in recording density, the wavelength of the recorded signal is proportionately reduced. Also, digital recording methods are starting to supplant the analog recording methods currently widely employed. In order to increase the recording density, a thin metallic film is sometimes used for the magnetic layer.
There have been many proposals to improve the electromagnetic characteristics of a coated magnetic tape in which one side of a substrate (nonmagnetic support) is coated with magnetic material. Some of these proposals relate to improving the magnetic properties of a ferromagnetic powder used in the magnetic material, while others relate to techniques for smoothing the surface of the magnetic layer. However, those proposals are still insufficient in satisfying the required high density of recording.
As the wavelength of the recorded signal is shortened due to the increased recording density, self-demagnetization sometimes occurs in the recording mode whereby the output level is reduced as the magnetic layer becomes thicker. For this reason, efforts have been made to reduce the thickness of the magnetic layer. However, when the magnetic layer is thinned, the surface of the magnetic layer can be more easily influenced by the surface roughness of the substrate. This results in deterioration of the electromagnetic characteristics of the tape.
To avoid this, production of magnetic tape with an extremely smooth surface is desired. A general means to realize this is to smooth the surface of the substrate. The smoothed substrate surface improves the electromagnetic characteristics of the magnetic layer formed on the substrate, but causes poor running of the magnetic tape constructed using such a substrate. To solve this problem, a back layer can be provided on the surface of the substrate opposite the surface thereof having the magnetic layer. For the details of a technique using a back layer, reference is made to Examined Japanese Patent Application No. Sho. 50-3927, and Published Unexamined Japanese Patent Application Nos. Sho. 57-111828, 52-102204, and 52-96505.
On the other hand, numerous problems arise from the provision of the back layer on the substrate surface. Any unevenness of the back layer surface physically influences the magnetic layer, impairing the electromagnetic characteristics of the magnetic layer. Moreover, as additional processes must be used in manufacturing the tape, the manufacturing defect rate increases, resulting in lowered productivity.
In order to reduce costs, mechanical parts used in common magnetic tape cassettes, such as magnetic tape cassettes of the .beta. type or the VHS type, are often manufactured with less precision. This creates problems in that the magnetic tape is more frequently subject to scratching, resulting in increased rates of dropout and poor tape running.
The fact that improving the surface property of the substrate improves the electromagnetic characteristics of the magnetic tape is well known. However, excessive improvement of the surface property impairs the tape running performance and durability. Also, the fact that increasing the coefficient of friction between the substrate surface of a magnetic tape 17 and tape guides 13 disposed in the cassette half as shown in FIG. 6 hinders the tape running is generally known.
In the cassette 10 shown in FIG. 6 where the magnetic tape 17 runs between a supply reel 11 and a take-up reel 12, a guide structure for guiding the tape to the forward side of the cassette, which structure includes cylindrical tape guides 13, a guide pole 14, a guide roller 15, and a pressure pad 16, is provided. The slide property of the guide structure and the magnetic tape, the mechanical frictional resistance therebetween, and the like determine the running performance of the magnetic tape 17. An additional factor affecting the tape running performance is triboelectrification (static electric charging) caused by the friction between the tape guides 13 and the magnetic tape 17. Particularly in the initial stage of tape running, an increase of triboelectrification causes the tape guide to generate an attraction force. The attraction force makes the tape running unstable. In an extreme case, the tape may be stopped altogether. At high potentials of static electricity, the attraction force creates a large friction between the substrate surface and the tape guides 13. The substrate surface can then more easily be scratched by the guides. When scratched, further larger static electricity is generated, and the attraction force is further increased.
To handle the electrification matter, USP 3,440,091 discloses a technique involving inserting a conductive layer between the magnetic layer and the substrate. A technique in which the surface electrical resistance of the magnetic layer is reduced to better handle the electrification problem is also known. For the latter technique, reference can be made to Published Unexamined Japanese Patent Application No. Sho. 60-70519.
In a magnetic tape having a low surface electric resistance of the magnetic layer and in which the conductive layer is used for the medium layer, if the tape does not include the back layer, the charge potential on the substrate surface increases with increases of the surface electric resistance of the substrate surface, the potential difference between the tape and the tape guides 13 increases, and consequently the tape runs unstably as mentioned above.
In a video tape cassette having tape guide members, such as the tape guides 13, it is desirable to obtain good tape running performance without providing the back layer. This contributes to cost reduction.
Various techniques for producing the tape guides 13 have been proposed, for example, in Published Unexamined Japanese Patent Application Nos. Sho. 61-192091 and 62-192092, and Published Unexamined Japanese Utility Model Application Nos. Sho. 61-136384 and 62-168136.
In Published Unexamined Japanese Utility Model Application No. Sho. 61-136384, an anodic oxidation surface layer is formed on the tape guide member of aluminum. The surface layer is worked by grinding to have fine linear grooves of specific depth (0.2 to 0.6 .mu.m) extending in the tape running direction. The publication describes that the durability of the tape guide is improved, scratching of the tape is reduced, and productivity of the magnetic tape is improved.
In the magnetic tape cassette disclosed in Published Unexamined Japanese Utility Model Application No. Sho. 62-168136, to manufacture the tape guide, a number of grooves are randomly formed in an aluminum surface. The resultant surface is hard chrome plated, and the surface is flattened by grinding.
Thus, the surfaces of the tape guide members disclosed in Published Unexamined Japanese Utility Model Application Nos. Sho. 61-136384 and 62-168136 are covered with an anodic oxidation layer and hard chrome plating layer. Aluminum, which is easy to work and low in cost, is used for the tape guide member. However, an additional step of forming the surface layer must be carried out. Moreover, this technique does nothing to ameliorate the triboelectrification problem and the attendant tape running problem.
Published Unexamined Japanese Patent Application No. Sho. 61-192691 discloses a magnetic tape cassette in which 520 to 700/2 mm.sup.2 number of protrusions of 0.0153 .mu.m or lower in height are formed on the surface of a tape guide which is brought into contact with the magnetic tape. The publication defines merely the number required of protrusions of a specific height. The height measurement is generally correlated with the surface roughness.
The conventional reference for the surface roughness measurement is based on the center line of the protrusions. However, the center line moves vertically depending on the size of the downward protrusions. The downward protrusions do not contact the magnetic tape. Nevertheless, these downward protrusions determine the surface roughness (the concept of the upward protrusions). In this respect, the conventional concept of surface roughness cannot provide a true criterion for defining the tape running performance. Data, plotted on graphs, of the relationship of take-up torque and the number of protrusions that are defined referring to the criterion, i.e., the number-of-protrusions vs. dropout relationship, and the relationship between the number of protrusions and substrate scratches, vary greatly. The above publication numerically defines the number of protrusions (upward protrusions) determined referring to this imperfect criterion. It is evident that the resultant product produced using such a technique is inherently unstable. Moreover, the publication teaches nothing related to the tape running characteristics in connection with triboelectrification.
Published Unexamined Japanese Patent Application No. Sho. 61-192092 discloses a magnetic tape cassette in which the radius of the top of a protrusion on the surface of the tape guide where it slidably contacts the magnetic tape is 25 to 100 .mu.m. The publication merely defines the sharpness of the tops of the protrusions. Formation of protrusions each with such a sharp top is impossible though as a practical matter. It is safe to say that the protrusions, even if so formed, cannot provide a tape guide of stable characteristics. This publication also teaches nothing regarding how tape running is affected by triboelectrification.
None of the above clearly describes the causes of attraction due to triboelectrification, but simply treat the electrostatic attraction as a factor involved in the increase of the friction force.
In Published Unexamined Japanese Utility Model Application No. Sho. 61-187089, the guide member is made of synthetic resin containing conductive material, and its surface electric resistance is set to a specific value or smaller. However, studies by the present inventors show that merely reducing the surface electrical resistance fails to provide the desired antistatic effect.
To increase the conductivity of the substrate, some additional measure, for example, mixing conductive filler into the material of the substrate or the magnetic layer, or formation of the back layer as referred to above, is required. As a result, the manufacturing process of the magnetic tapes is made complicated, but the electrostatic attraction problem still is not completely solved.
Soviet Patent SU 128119A discloses a magnetic tape cassette in which, to prevent triboelectrification between the tape guide and the magnetic tape, a grounding pin is connected to the tape guide. This proposal effectively prevents triboelectrification. However, the use of the grounding pin makes the cassette construction complicated. To secure good grounding, means outside the cassette must be used additionally.